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Ultrafast ultrasonic imaging coupled to rheometry: principle and illustrationWe describe a technique coupling standard rheology and ultrasonic imaging with promising applications to characterization of soft materials under shear. Plane wave imaging using an ultrafast scanner allows to follow the local dynamics of fluids sheared between two concentric cylinders with frame rates as high as 10,000 images per second, while simultaneously monitoring the shear rate, shear stress, and viscosity as a function of time. The capacities of this -rheo-ultrasound- instrument are illustrated on two examples: i the classical case of the Taylor-Couette instability in a simple viscous fluid and ii the unstable shear-banded flow of a non-Newtonian wormlike micellar solution.